Contents

There are forty three segments of nerves in our body and with each segment there is a pair of sensory and motor nerves. In the body, thirty one segments of nerves are in the spinal cord and twelve are in the brain stem.

Besides these, thousands of association nerves are also present in the body.

Thus Somatic Nervous System consists of three parts:

i) Spinal Nerves: They are peripheral nerves that carry sensory information into the spinal cord and motor commands.

ii) Cranial Nerves: They are the nerve fibers which carry information into and out of the brain stem. They include smell, vision, eye, eye muscles, mouth, vision,. Taste, ear, neck, shoulders and tongue.

From there, acetylcholine is released from the axon terminal knobs of alpha motor neurons and received by postsynaptic receptors (Nicotinic acetylcholine receptors) of muscles, thereby relaying the stimulus to contract muscle fibers.

In invertebrates, depending on the neurotransmitter released and the type of receptor it binds, the response in the muscle fiber could either be excitatory or inhibitory. For vertebrates, however, the response of a muscle fiber to a neurotransmitter (always acetylcholine (ACh)) can only be excitatory.

A reflex arc is a neural circuit that creates a more or less automatic link between a sensory input and a specific motor output. Reflex circuits vary in complexity—the simplest spinal reflexes are mediated by a three-element chain, beginning with sensory neurons which activate interneurons in the spinal cord, which then activate motor neurons. Some reflex responses, such as withdrawing the hand after touching a hot surface, are protective, but others, such as the patellar reflex "knee jerk" activated by tapping the patellar tendon, contribute to ordinary behaviour.